It will be appreciated that Westinghouse Air Brake Company air dryers are designed to remove moisture from compressed air for use in compressed air systems as utilized on railway trains, transit vehicles and the like, as necessary to operate the air brake system and air valves. U.S. Pat. No. 5,423,129, assigned to the assignee of this invention and cited above, discloses such a system wherein the compressed air is cleaned and dried by passing it through a regenerating system containing a desiccant material to absorb the moisture and filter-out particulate matter. The desiccant is regenerated by passing a small percentage of the dried air back therethrough which absorbs at least some of the moisture collected in the desiccant and is therefore discarded.
In operation, the above noted prior art air drying system (now referred to as the "single tower" system) receives compressed air from a conventional air compressor, a supply which typically contains an unacceptably high amount of moisture and other particulates suspended therein. This unpurified compressed air is caused to flow upwardly through a desiccant material usually in the form of a porous cartridge containing a porous desiccant media. The desiccant plays the key role within the single tower air drying system in that it absorbs the moisture and traps various particulates (e.g., dust, dirt, etc.) as the compressed air moves upwardly through the desiccant media. Once moisture and particulates are extracted from the air stream, the cleaned and dried compressed air continues flowing from the desiccant media through a purge check valve situated near the top of the tower. This purified compressed air then passes through a side chamber, a part of which eventually reaches a purge volume.
When the air compressor is cycled off, the system operates in a purge mode. During the purge mode, the purified pressurized air contained within the purge volume passes slowly in the reverse direction through a choke in a purge check valve and then back through the desiccant media. This slow stream of dried air reabsorbs a portion of the moisture previously collected within the desiccant media. Having evaporated into this passing stream of dry air, the evaporated moisture eventually exhausts through the sump volume to atmosphere. This gradual purging of dry air back through the system serves to dry out and thus rejuvenate or regenerate the desiccant media. When the air compressor is again cycled on, the tower system returns to operation in a drying mode with the desiccant media then again removing moisture from the stream of unpurified compressed air passing therethrough.
More recently, a twin tower system has been proposed and developed in which a pair of desiccant containing chambers or towers are provided, each alternating back and forth between operation in drying mode and in recycle mode. Accordingly, at any given moment of operation, one tower is operating in air drying cycle while the other is operating in recycle mode or purge cycle. A pair of control valves are provided to automatically switch the flow to reverse these flow directions so that after a defined time period the cycles are reversed so that in effect a continuous operation is achieved with each tower alternately operating in drying mode permitting moisture to collect within the desiccant media while the other tower is in recycle mode removing the collected moisture from the desiccant material or media. This unique system obviously has a greater moisture removing capability and also avoids the need to have the source of unpurified air cycled-off in order to purge the desiccant material of the moisture it has accumulated and thereby eliminate the need to temporarily deprive the pneumatic system of a steady supply of clean and dried compressed air while the compressor is turned off.
In addition to the above advantages, the switching of the two drying assemblies alternately between the drying and the purging modes allows the twin tower system to exsiccate the air stream more efficiently than the prior art single tower system. Two desiccant towers rather than one are employed in the air drying system with one absorbing moisture while the other is being purged of it. The switching of the two drying assemblies alternately between the drying and the purging modes thus serves to continuously purge moisture from the twin tower system. More fully desiccated air is thus supplied to the pneumatic system. The amount, density and overall surface area of the desiccant can also be selected to suit varying needs.
The twin tower system can be applied to a wide variety of pneumatic systems. Typical of the types of pneumatic systems to which the twin tower system could be applied include the pneumatic brake systems of passenger and freight railroad trains, subway trains and various other types of rail related transportation systems. Further examples include the pneumatic brake systems of various truck transport vehicles. Other types of pneumatic systems to which the twin tower system could be applied may be found outside the transportation field.
Another disadvantage of the single tower air drying system is that it is only capable of removing a certain, limited amount of moisture during the purge mode. Because the volume of unpurified air flowing into the system to be dried vastly exceeds the volume of purified air used to purge the desiccant media, the desiccant media never adequately exsiccates during operation of the single tower system. Indeed, the desiccant media adequately exsiccates only after the system has been turned off for a prolonged time sufficient to accomplish same.